A radio frequency (RF) identification system consists of an RF base station and one or RF tags. In a typical configuration, the base station (also referred to as a reader) has a computer section which issues commands to an RF transmitter and receives commands from an RF receiver. The commands serve to identify tags present in the RF field or range of the base station. In some implementations, base station commands exist to gather (read) tag information from the tags after the tags in the field are identified. In more advanced systems, once the tags in the field are identified, base station commands exist which output (write) information to the tags. This output information may be held temporarily on the tag, it may remain until over written, or it may remain permanently on the tag.
The RF transmitter of the base station encodes the command from the computer section. The encoded command is then modulated from a base band signal on the radio (carrier) frequency. The modulated carrier is amplified by the base station and passed to a base station RF antenna for transmission to one or more RF tags in the base station field. The tags transmit or reflect a return signal back to the base station, sometimes with tag information encoded on the return signal by the tags. The base station RF receiver gathers (reads) the return signal at the base station RF antenna, demodulates the return signal from the RF carrier frequency to the base band, decodes the base band signal, and passes the decoded base band signal (information) back to the computer section for processing. The base station antenna sends RF signals to and receives RF (return) signals from one or more tags within the RF signal range. The tags within the range of the RF carrier frequency are said to be in the field of the base station. See U.S. Pat. No. 4,656,463 to Anders et al. entitled LIMIS Systems, Devices and Methods, issued on Apr. 7, 1987 which is herein incorporated by reference in its entirety.
In these prior art systems, the base station must identify some or all of the tags in the field of the base station (reader) before any data can be written to any of the tags in the field. In one prior art system, tags are temporarily turned off (deactivated) once the tags are successfully identified by the reader. After a period of time, e.g. approximately 10 minutes, the deactivated tags can be reactivated. See EP 0 494 114 to Marsh et al. filed on Mar. 1, 1992 and entitled “Electronic Identification System” which is herein incorporated by reference in its entirety.